Same reason you don't see Socket Rings/Triple Socket items/Socket(Double) Necks etc. as often in the beginning tier of expansion gear. There needs to be room for the gear to grow socket wise as there are more tiers of gear available.

To get do a more theoretic answer, item level is a function of total allocated stats for a set number. So 500 ilvl piece of gear has a certain "budget" as it were. Think of it as a simcity game,you spend money on roads you have less to build houses with. Likewise, if a piece has a socket, it has less of a budget for more strength or secondary stats. Likewise, the more secondary stats a piece has, the less primary stats. This is not a hard and fast rule, necessarily. The reason the heroic cloak off Lord Ryloith in Firelands was still BiS for hunters in heroic DS was because the 391 cloak was over budgeted, since it had 3 sockets (And epic gems were introduced, meaning that meant 30 more agility) and maybe the other stats were over budgeted too (I forget). But this is why you may see a blue with no socket having higher strength than a purple with sockets,or another blue with sockets.